Subclass responses and their half-lives for antibodies against EBA175 and PfRh2 in naturally acquired immunity against Plasmodium falciparum malaria

Abstract

Background: Plasmodium falciparum EBA175 and PfRh2 belong to two main families involved in parasite invasion, and both are potential vaccine candidates. Current knowledge is limited regarding which target antigens and subclasses of antibodies are actually important for protection, and how naturally acquired immunity is achieved.

Methods: Repeated blood samples were collected from individuals in Nigeria over a period of almost one year. ELISA was used to analyse subclasses of IgG responses.

Results: For both EBA175 (region III-V) and (a fragment of) PfRh2, the dominant antibody responses consisted of IgG1 and IgG3 followed by IgG2, while for PfRh2 there was also a relatively prominent response for IgG4. High levels of IgG1, IgG2 and IgG3 for EBA175 and total IgG for PfRh2 correlated significantly with a lower parasitaemia during the study period. Children with HbAS had higher levels of some subclasses compared to children with HbAA, while in adults the pattern was the opposite. The half-lives of IgG2 and IgG4 against EBA175 were clearly shorter than those for IgG1 and IgG3.

Conclusion: EBA175 and PfRh2 are potential targets for protective antibodies since both correlated with lower parasitaemia. The shorter half-lives for IgG2 and IgG4 might explain why these subclasses are often considered less important in protection against malaria. Triggering the right subclass responses could be of critical importance in a successful vaccine. Further studies are needed to evaluate the role of haemoglobin polymorphisms and their malaria protective effects in this process.

Figure 1

Total IgG responses to EBA175 in individuals five to 70 years old (number of individuals = 40) from July 2009 to June 2010 (number of samples = 302). The bars show mean ± SEM optical density (OD) value at 414 nm. (April not shown, no sample collection). (Black bars = rainy season; white bars = dry season). PfRh2: Data not shown.

Figure 2

IgG subclass responses to Plasmodium falciparum merozoite antigens (A) EBA175 and (B) PfRh2 in individuals five to 70 years old, followed over almost one year. The scatter plot shows mean ± SEM OD for values at 414 nm; all samples (n = 302) from all individuals (n = 40) are included.

Figure 3

Parasitaemia in individuals (n = 39) five to 70 years old from July 2009 to June 2010. The bars show mean ± SEM parasitaemia (parasites/μl). (April not shown, no sample collection; June: one individual who had very high parasitaemia, 15 168 parasites/μl, was excluded from this figure. (Black bars = rainy season, white bars = dry season).

Figure 4

The total IgG and subclass levels to EBA175 (A, B) and PfRh2 (C, D) in HbAA and HbAS for children (n = 18) and adults (n = 22) are shown; the number of samples = 302. The bars show mean ± SEM of OD at 414 nm. (*, p <0.05; **, p <0.01; ***, p <0.001).

Figure 5

The half-lives of IgG2 and IgG4 were shorter than those for IgG1 and IgG3.